1. Field of the Invention
The present invention relates to a pressure and temperature transducer suitable for use downhole in oil, gas, geothermal and other wells, at the wellhead, in industrial applications, for portable calibration devices and in laboratory applications. More specifically, by way of example and not limitation, the invention in its preferred embodiment relates to a piezoelectrically-driven quartz crystal resonator pressure and temperature transducer exhibiting improved thermal coupling to the environment external to a housing in which the transducer is located. The invention provides enhanced transient response for the transducer to better accommodate relatively rapidly-changing temperatures external to the housing such as may be encountered when the transducer is rapidly run into, or withdrawn from, a deep well bore. The invention also provides enhanced transient response for the transducer to better accommodate rapidly changing temperatures in a pressure-transmitting fluid inside the housing, such as those initiated due to rapid changes in pressure (commonly referred to as PV heating) such as may be produced by fluid sampling for formation testing or other rapid pressure drawdowns in a well bore.
2. State of the Art
The type of quartz crystal pressure transducer assembly in which sensors, as disclosed herein may preferably be employed includes a first pressure sensitive quartz crystal resonator, a second temperature sensitive quartz crystal resonator, a third reference frequency quartz crystal resonator, and supporting electronics. For convenience, the terms "crystal" and "resonator" may be used interchangeably herein in referencing a resonating quartz crystal element.
In a transducer assembly of the referenced type, the first crystal changes frequency in response to changes in applied external pressure and temperature, while the output frequency of the second crystal is used to compensate for temperature-induced frequency excursions in the first and third crystals. The third crystal generates a reference signal, which is only slightly temperature dependent, against or relative to which the temperature and pressure-induced frequency changes in the first crystal and the temperature-induced frequency changes of the second crystal can be compared. Means for comparison, as known in the art, include frequency mixing and/or using the reference frequency to count the signals from the other two crystals. The first resonator is exposed via a fluid interface to the external pressure sought to be measured, and all three resonators are preferably thermally coupled to the fluid to provide a rapid thermal response time. The transducer (crystals plus electronics, the latter disposed in a pressure housing) is calibrated as a complete unit over the intended pressure and temperature range so that all temperature and pressure related effects can be compensated for in the resulting calibration curve-fit coefficients. Exemplary patents for transducers using three crystal resonators, each assigned a function as described above, are U.S. Pat. No. 3,355,949 to Elwood et al., U.S. Pat. No. 4,802,370 to EerNisse et al. and U.S. Pat. No. 5,231,880 to Ward et al.
The first crystal, which may also be termed a pressure crystal or pressure sensor crystal, employed in pressure transducer assemblies of the prior art, has been commonly configured to include a disc-shaped resonator element incorporated in a tubular cylindrical housing assembly, the ends of the housing assembly being closed. The cylindrical housing assembly, when subjected to exterior pressure of a fluid to be monitored, elastically deforms and thus causes the frequency of the resonator element to shift, the frequency output thus being indicative of the pressure. As noted above, the frequency output may then be preferably temperature-compensated, as known in the art. Exemplary pressure sensor crystal configurations are disclosed in U.S. Pat. No. 3,561,832 to Karrer et al., U.S. Pat. No. 3,617,780 to Benjaminson et al., U.S. Pat. No. 4,550,610 to EerNisse, U.S. Pat. No. 4,660,420 to EerNisse, U.S. Pat. No. 4,754,646 to EerNisse et al., U.S. Pat. No. 4,802,370 to EerNisse et al., U.S. Pat. No. 5,221,873 to Totty et al., U.S. Pat. No. 5,578,759 to Clayton and in EerNisse, "Quartz Resonator Pressure Gauge: Design and Fabrication Technology," Sandia Laboratories Report No. SAND78-2264, (1978).
U.S. Pat. No. 4,660,420 to EerNisse recognizes the desirability of selecting a pressure crystal with a crystal cut having substantial independence from temperature-induced frequency changes over the intended range of temperatures, as well as a relatively large scale factor, i.e., greater frequency sensitivity to pressure changes in the range to be measured. For the pressure and temperature ranges experienced in oil and gas wells, an AT-cut quartz crystal is disclosed in EerNisse '420 to possess these attributes. However, even the AT-cut quartz crystal disclosed in EerNisse '420 exhibits some demonstrated susceptibility to temperature changes.
It is desirable to electrically insulate the first, or pressure sensor, crystal from the interior wall of the metal pressure housing in which the crystal is located. As disclosed in the aforementioned U.S. Pat. Nos. 5,231,880 and 5,578,759, a fairly robust electrically insulating spacer has typically been placed about the pressure sensor crystal. An exemplary prior art transducer sensor arrangement similar to that shown in these two patents is reproduced in FIG. 1 of the drawings, wherein transducer 10 includes electronics which drive and respond to the output of quartz crystal resonators (see FIG. 2). The electronics and resonators are usually contained within a common housing, such as pressure housing 12. The three resonators, as mentioned above, include a pressure crystal 14, a temperature crystal 16 and a reference crystal 18. Only the pressure crystal 14 is subjected to the pressure of fluid external to the pressure housing 12 via inlet 30, temperature crystal 16 and reference crystal 18 being intentionally isolated from pressure effects by their packaging and location. The electrical feedthrough (not shown for clarity) to electronics in chamber 32 for driving and sensing the frequency response of pressure crystal 14 to changes in pressure is pressure proof, as known in the art. Cup-shaped electrically insulating spacer 34 may be formed of a plastic such as polyetheretherketone (PEEK) and typically has a side wall 34a of thickness T of about 70 mils, or 0.070 inch. Spacer 34 supports the pressure crystal 14 while permitting access by a fluid 35 surrounding the crystal to substantially the entire exterior thereof so as to enable substantially instantaneous transmission to the crystal of pressure external to the pressure housing. The spacer 34 also facilitates flushing contaminated fluids from around pressure crystal 14. However, a preferred bellows-type pressure transmission configuration which isolates fluid 35 from the environment external to the pressure housing, as well as preferred materials for fluid 35, is disclosed in U.S. Pat. No. 5,337,612 to Evans, the disclosure of which is incorporated herein for all purposes by this reference. The invention of the '612 patent has substantially eliminated any need for flushing. Protection of pressure crystal 14 against lateral shock loading is effected through the phenomenon of "squeeze-film damping" in the two coaxial fluid-filled annular spaces 36 and 38, respectively, lying between pressure crystal 14 and spacer side wall 34a and spacer side wall 34a and inner wall 40 of pressure housing 12. A thin circular disc 39 of an electrically insulating material such as Kapton.RTM. may be placed at the open end of spacer 34 to protect one end of pressure crystal 14 against axial shock loading (again, by squeeze-film damping) and electrical shorting, while the end wall 34b of spacer 34 protects the other end of pressure crystal 14. Spacer 34 also provides diametrically-opposed longitudinal channels (not shown) to accommodate wires running the full length of pressure crystal 14 to the electrical feedthrough from wire attachment points on the sides of pressure crystal 14 proximate end wall 34b to provide slack and flexibility to the wires and reduce stress on the joints (using, for example, conductive epoxy or solder) securing the wires at the wire attachment points to plated, electrically-conductive electrodes on the crystal exterior. Spacer 34 also precludes electrical grounding of the electrodes, wire bonds and any bare wire portions to inner wall 40 of pressure housing 12. However, while providing some benefits to the structure of transducer 10 as configured, the relatively thick spacer side wall 34a precludes effective thermal coupling of pressure crystal 14 with pressure housing 12 under rapidly transient temperature conditions such as might be encountered when running transducer 10 on a wire line or slick line into or out of a well bore at a relatively high rate of speed.
Similarly, the disposition of relatively thick, electrically insulating end caps over a sensor assembly, such a structure being disclosed in the aforementioned U.S. Pat. No. 4,802,370, is accompanied by the same disadvantages as mentioned above with respect to the use of a spacer.
To quantify the temperature transients involved under conditions wherein a transducer is moving in a well bore, 1.degree. C. per minute may be characterized as typical, while 2.degree. C. per minute may be close to the maximum. Such temperature changes external to the pressure housing may induce error in indicated pressure. In addition, pressure steps also produce temperature changes due to PV heating (see U.S. Pat. No. 5,471,882); a 4000 psi step can produce a 1.degree. C. step in the fluid surrounding the pressure crystal.
U.S. Pat. Nos. 5,337,612 and 5,471,882 disclose transducer assemblies which do not depict or discuss the use of a spacer, but which nonetheless depict a significant annular gap or void between the pressure crystals (or pressure and temperature in the case of the '882 patent), which gap or void would significantly reduce thermal coupling between the crystals and the surrounding pressure housing. Further, the large volume of fluid surrounding the pressure crystal in the pressure housing cavity would render the transducer susceptible to significant PV heating. Moreover, while a spacer was not depicted or discussed in the '612 or '882 patents as not material to the inventions respectively disclosed and claimed therein, in actuality, transducers according to the respective inventions of the '612 and '882 patents were always built with spacers, most commonly of the aforementioned PEEK material but, in a small number of cases, of alumina. An electrically insulating spacer was always required to prevent electrical grounding of the pressure crystal to the metal pressure housing.
While prior art devices, as referenced above, have attempted to address various deficiencies in quartz resonator and transducer design, those of ordinary skill in the art have failed to recognize that thermal coupling and thus transient temperature response of such transducers may be greatly enhanced and inaccuracy reduced in the presence of rapidly-changing temperatures external to the transducer pressure housing through certain relatively straightforward modifications to the physical structure of the transducer assembly and resonators employed therein.